RF amplifiers and other high frequency electronic apparatus, are typically constructed today by combining various solid state amplifying devices, e.g., transistors, with other passive and active components on a circuit board. The circuit board is generally an insulating substrate on which are formed various flat metallic traces which serve to interconnect the various components on the board and deliver input signals, output signals, power and ground. Such circuit boards are typically formed by (1) bonding or depositing continuous metal foils on an insulating fiberglass or ceramic substrate and then etching the foil into the desired metal traces or (2) printing conductive inks in the shape of the desired metal traces directly onto the insulating substrate and then heating to release the organic binders and solvents. Such circuit boards are referred to in the art as "printed circuit boards" or by the abbreviation "PC boards". The components are typically attached to the metal traces by soldering although other means well known in the art can also be used.
A problem common to most RF amplifiers and other high frequency apparatus is control of internal signal radiation which can cause feedback and cross-talk from one part of the circuit to another. This is especially true in circuits which must handle substantial amounts of RF power. Such amplifiers and apparatus usually have very stringent internal shielding requirements.
It has been common in the art for many years to use shielded cables to reduce cross-talk and spurious signal coupling. Shielded cables typically comprise a round outer metallic covering (i.e., the shield) surrounding one or more internal wires. The shield may be flexible, e.g., formed from a copper braid, or rigid, e.g., formed from a round copper tube. The internal wires may be embedded in a plastic matrix which fills the shield so that their position within the shield is fixed or they may be loose. Another approach to shielding has been to provide a metal frame or chassis around and/or beneath the circuit board and have channels formed in the metal frame or chassis through which the wires desired to be shielded are routed.
These prior art approaches suffer from a number of disadvantages well known in the art. For example, flexible, braided shield conductors are difficult to position precisely and retain in predetermined locations in the RF assembly. Movement of the shielded cable can adversely affect the circuit performance. While this problem is partially overcome by using shielded cables with rigid shields, prior art versions have used round tubular shields which are difficult to control in manufacturing because of their propensity to roll or move during assembly.
Routing of the wires to be shielded through channels provided in the amplifier frame or enclosure avoids the problems associated with free-standing shielded cables, but channels are comparatively much more expensive to manufacture. Thus, there is an ongoing need for further improvement in the means and methods used for providing shielded wires or cables in RF and other high frequency apparatus where internal signal leakage and wire placement must be controlled to a very high degree.
As used herein, the words "radio frequency" and the abbreviation "RF", and the words "high frequency", are intended to refer to frequencies about .gtoreq.10.sup.6 cps, typically about 10.sup.8 to 10.sup.10 cps, and the abbreviation "DC" is intended to refer to substantially direct current and the abbreviation "AC" is intended to refer to alternative current.